1. Field of the Invention
The present invention relates to an optical pickup for recording and reproducing information on and from a recording medium such as an optical card and an optical disc.
The present invention also relates to an optical pickup for reproducing information recorded on the recording medium, such as the optical card and the optical disc or the like, especially to a multi-track read type optical pick up for reproducing plural tracks, simultaneously.
2. Related Art Statement
As a conventional optical pickup, there is an optical pickup in which in order to perform tracking control by, for example, 3 beam method, or to reproduce plural tracks simultaneously, a light beam from a semiconductor laser is collimated by a collimator lens to form a parallel luminous flux, this flux is divided into a plurality of light beams by means of a diffraction grating, these light beams are irradiated with a predetermined positional relation through an inward and return way separating optical element and an objective lens, and these return light are lead to a detecting optical system by separating the return light from the inward way through the objective lens.
In such an optical pickup, in order to increase utility efficiency of the light beam from the semiconductor laser, it is advantageous to utilize a polarizing beam splitter as an inward and return way separating optical element. To this end, for example, in the inward way, it is necessary to dispose the semiconductor laser in such a manner that a polarizing plane of emitting light thereof is made a predetermined direction for the polarizing beam splitter.
While, even though the semiconductor laser is disposed in such a way, for example, in the case of shaping a light beam of an ellipsoidal section emitted from the semiconductor laser into a circular section by a beam shaping prism, the semiconductor laser can not be disposed in such a way. As a countermeasure in this case, it is conceived that the semiconductor laser is disposed so as to shape the emitted light in the beam shape having circular section by the beam shaping prism and the polarizing direction thereof can be rotated by 90.degree. by disposing a 1/2 wave plate between the semiconductor laser and the polarizing beam splitter.
In this case, however, as shown in FIG. 24, provided that a 1/2 wave plate 152 is disposed behind a diffraction grating 151 with an azimuth of 45.degree., an incident light beam Bin of linear polarization (inner direction in paper) from the semiconductor laser is incident on the 1/2 wave plate 152 by separating the light beam into, for example 0 ordered light B0 and .+-.1 ordered diffraction lights B+1, B-1 by the diffraction grating 151. Even though the 0 ordered light B0 is converted into a linear polarization having 90.degree. rotated polarization plane, the .+-.1 ordered diffraction lights B+1, B-1 are not linearly polarized under the influence of incident angle dependency of the 1/2 wave plate 152 and are converted into right-handed and left-handed ellipsoidal polarization. Therefore, in the latter polarization beam splitter (not shown), there is a problem that an optical property for the ii ordered diffraction lights B+1, B-1 is decreased and thus primary problem can not be attained. Such a problem becomes remarkable in case of obtaining more high ordered diffraction light, in the diffraction grating 151.
In the case that the recording semiconductor laser and the reproducing semiconductor laser are used, light beams from these semiconductor lasers are irradiated on the polarization beam splitter, light beam from one semiconductor laser is transmitted or reflected through or on the polarization beam splitter, and the light beam from the other semiconductor laser is reflected or transmitted to c ose them in the same optical path, and these light beams are irradiated on the recording medium through common objective lens, the above problem arises in the same manner even in case of separating light beam from at least one of semiconductor lasers into plural sub-beams and of irradiating them on the polarization beam splitter.
When the recording semiconductor laser and the reproducing semiconductor laser are used, the light beams from these semiconductor lasers are composed substantially in the same optical path by the polarization beam splitter, and are irradiated on the recording medium through the common objective lens, the disposition of both semiconductor lasers can be considered so as to make polarizing planes of respective emitted lights orthogonal.
In case of disposing both semiconductor lasers in such a way, when light beams of ellipsoidal sectional shape emitted from respective semiconductor lasers are not subjected to a beam shaping to obtain light beam of circular sectional shape, the direction of respective ellipsoidal spots formed on the recording medium have major axis direction orthogonal to the tracks in recording time, and have major axis direction parallel to the tracks in reproducing time, and thus become orthogonal to each other, so that the strength distribution of respective spots on the recording medium are not all the same. Therefore, resolution and resolving power are not all the same at recording time and reproducing time and thus recording and reproducing with high precision can not be performed.
As a conventional optical pickup, there is provided an optical pickup in which in order to follow, for example, a recording beam or a reproducing beam to a predetermined track of a recording medium precisely, to record information while verifying, and to reproduce the information recorded on plural tracks of the recording medium, a plurality of light beams are irradiated on the recording medium in the form of spot shape and in a predetermined positional relation, the light reflected on the recording medium is received on corresponding light receiving element, thereby detecting required signals.
However, in the above conventional optical pickup, in some recording mediums, there is a problem that the return lights of the light beam irradiated at an adjacent position of the recording medium surface are also incident on other light receiving elements, so that these return lights are subjected to interference phenomena with each other, thereby causing large amounts of cross-talk, and thus the required signals can not be detected with the requisite precision.
For example, as in an optical card 161 shown in FIG. 25, the recording medium formed by laminating an adhesive layer 163, a recording layer 164 and a transparent layer 165 on a substrate 162 of resin, successively, has 400 .mu.m thickness of the adhesive layer 163, a few tenths to a few hundredths nm thickness of the recording layer 164 and 400 .mu.m thickness of the transparent layer. When plural light beams are condensed on the recording layer 164 of such an optical card 161 through the transparent layer 165 in a predetermined positional relation so as not to with respective reflected lights, if the condensed points of adjacent light beams on the recording layer 164 are closed, a cross-talk for which the reflected light of one light beam reflected on a boundary plane between the adhesive layer 163 and the substrate 162 through the recording layer 164 is entered into the return light of adjacent other light beams, thereby affecting unfavorable influence on the detection of the required signal. Moreover, in the case shown in FIG. 25, adjacent light beams irradiated on the optical card 161 are reflected somewhat on the surface of the transparent layer 165, so that the return lights thereof are interfered with each other on the corresponding light receiving element. However, the surface of the transparent layer 165 is sufficiently separated from the recording layer 164, so that the return lights on the surface are sufficiently diffused, and thus there is substantially no problem.
As the conventional optical pickup, there are two types of pickups; one is an integrated unit type capable of moving whole unit in the seeking direction crossing tracks of a recording medium, and the other is a separation type capable of moving a part of the optical pickup. In either type of optical pickup, optical members which form the optical pickup must be disposed with precise positioning.
In the conventional optical pickup, it is proposed that the optical members are held elastically in such a manner that the members can be moved in a predetermined direction being the adjusting direction, for example, in an optical axis direction, cannot be moved in the other direction, for example, in the direction orthogonal to the optical axis so as to adjust the position of the members in the optical axis direction. In such a way, the position of the optical members can easily be adjusted in the predetermined direction without affecting the position in the other direction.
However, in the conventional optical pickup, the relation of the adjusting direction of the optical members and the seeking direction the read is not considered at all. Therefore, in case of holding the optical member including in the movable pickup portion (hereinafter, referred to as movable member) elastically as described above so as to make the positional adjusting direction of the optical member and the seeking direction being the moving direction of the movable member parallel, if the movable member is run away in the seeking direction in any cause, its acceleration or the padding of the movable member to the run away stopper shift in the positional adjusting direction, so that desired optical characteristic can not be obtained.
For example, in the means for reading the optical card, the relative moving speed of the optical card and the optical pickup is slower than that of the means for reproducing or playing back the disc-shaped record medium, such as the CD or the like. Therefore, the reading beam is irradiated to each or respective plural tracks through an objective lens to receive the reflecting beam by the separate light receiving element so as to reproduce or playback the plural tracks simultaneously with only one scanning. Furthermore, it is necessary to follow the optical pickup, which is not only the above multi-track read type, to the track reading the beam for reading in the focusing state exactly. Accordingly, for instance, two other tracking beams in addition to the reading beam are irradiated to the recording medium to control the tracking by detecting the tracking error signal through the three beams method based on those reflected lights and to control the focus by detecting the focus error signal in addition to the playing signal based on the reading beam which is reflected by the record medium.
FIG. 26 describes an example of the relative positional relation between the spot formed on the record medium by the plural light beams carrying out the above multi-truck read and the track. Regarding this optical pickup, three reading beams and two tracking beams are irradiated on the optical card 171 to read three tracks successively. Three reading beams are irradiated to locate each spot 172a, 172b, 172c on successive three tracks 173a, 173b, 173c of the optical card 171. Two tracking beams are irradiated to locate each spot 174a, 174b on one side edge and the other side edge of different guide luck 175a, 175b.
FIG. 27 describes the constitution of the light detector receiving the reflected light on the optical card 171 of each light beam, arranged on the optical pickup carrying out the multi-truck read through detecting the plural light beams shown in FIG. 26. This light detector 181 has light receiving element 182a, 182b, 182c receiving each reflected light of reading spots 172a, 172b, 172c independently and light receiving element 183a, 183b receiving each reflected light of tracking spot 174a, 174b independently. In this situation, for example, the light receiving element 182b is constituted of the light receiving range divided into four parts. Regarding said light pickup, based on the output of the light receiving element 182a, 182b, 182c, the reading signal of the corresponding truck 173a, 173b, 173c is obtained and the trucking error signal is obtained through the three beams method based on the output of the light receiving element 183a, 183b. Furthermore, based on the output of the four-divided light receiving range of the light receiving element 182b, for instance, the focus error signal is obtained by the astigmatism means. Moreover, the reading signal of the truck 173b is obtained through adding the output of each light receiving range of the light receiving element 182b.
Concerning the above multi-truck reading light pickup, although the plural light beams irradiating to the record medium can be obtained by using each independent light source, for instance the semiconductor laser, a large and expensive device will be needed so that generally using one semiconductor laser, said laser beam is diffracted with plural diffraction gratings so as to obtain requested plural light beams. For example, to obtain five light beams shown in FIG. 26, using the first diffraction grating, which diffracts the light beam from one semiconductor laser and second diffraction grating, which diffracts the light beam, passed said first diffraction grating, to a different direction from the first diffraction grating direction, three light beams, which are 0 ordered diffraction light and .+-.1 ordered diffraction lights, are obtained. Each 0 ordered diffraction light on the second diffraction grating of said three light beams is the beam for reading and a pair of .+-.1 ordered diffraction lights on the second diffraction grating is the tracking beam.
At this point, based on the output of each light receiving element of the light detector 181 shown in FIG. 27, to detect the reading signal, the focus error signal and the trucking error signal, generally the output electric current of the light receiving elements 182a, 182c, 183a, 183b are successively changed to the voltage. Regarding the light receiving element 182b, the output current of each light receiving range is converted into the voltage so as to treat or after treating with the voltage mode, current to voltage conversion is carried out. In this case, as regards the circuit constitution and the cost or the like, it is favorable to use the I/V conversion, which accumulates the plural I/V conversions, having a relatively narrow dynamic range and the operating circuit, and the operator.
Therefore, with regard to above multi-truck reading light pick up, in FIG. 26, the light amount ratio of 0 diffraction light to .+-.1 ordered diffracting lights at the first diffraction grating is set up so that the light quantity ratio of three reading beams for forming the reading spot 172a, 172b, 172c is 1:1:1. Furthermore, since the light receiving element 182b is divided into four parts to detect the focus error signal also, the light quantity ratio of -1 ordered diffraction light, 0 ordered diffraction light and +1 ordered diffraction light on the second diffraction grating is 1:4:1 so that the light quantity, which in incident upon one of the light receiving ranges, and the light amount of the tracking beam, which respectively incidents upon the light receiving element 183a, 183b for detecting the tracking error signal, is nearly equal.
On the other hand the, above focus control and tracking control is carried out not only on the read-type (reproducing only) light pick up but on the write type (recording-reproducing) as well. There is known that as a write-type light pick up, using light beam from the semiconductor laser through one piece of diffraction grating, three light beams of 0 ordered light and .+-.1 ordered diffraction lights is obtained, then said one 0 ordered light is irradiated upon the track of the record medium as a beam for writing, said reflected light is received by the light receiving element having four-divided light receiving range as well as shown in FIG. 27 so that the focus error signal is detected through the astigmatism method and .+-.1 ordered diffraction lights are irradiated to one side of the edge and the other side of the edge of different guide track as shown in FIG. 26 so as to receive each reflected light by light receiving element respectively as shown in FIG. 27 to detect the tracking error signal by 3 beams method.
Regarding said write-type light pick up, since the emerging power of the semiconductor on writing is over ten times as large as that of the read type one, the light amount ratio of -1 ordered diffraction light, 0 ordered diffraction light and +1 ordered diffraction light is set up, for example, 1:16:1 to certainly prevent the not-requested entry by the beam for tracking.
Incidentally, to produce above multi-track read type light pickup and the write type light pickup, it is desirable to use common constructing member and manufacturing member from the point of the cost or the like. Especially, with regard to the diffraction grating it is unfavorable to manufacture the exclusive mask pattern and the exclusive diffraction grating for the expensiveness.
However, when the diffraction grating used in above write type light pick up is used as the second diffraction grating for above multi-track read type light pick up, in FIG. 27, the light quantity which incidents to the light receiving element 183a, 183b for the tracking error signal detection becomes extremely small since the emerging power of the semiconductor laser for the read type light pickup is small. Therefore, when the I/V converter and the operator, which dynamic range is narrow, is used as written above, the tracking error signal can not be precisely detected. To solve this problem, using the I/V conversion and the operator, which dynamic ranges are broad, or using the exclusive I/V converter and the exclusive operator, which have high sensitivity against the light receiving element 183a, 183b may be considered. In both cases, the result is higher manufacturing costs. In the latter cases, there is a problem that the constitution of the circuit becomes complicated.
Conversely, it is possible to use the second diffraction grating, which is used in the multi-track read type optical pickup, as the write type optical pickup. Regarding above second diffraction grating, the light amount ratio of the 0 ordered light and the +1 ordered diffraction light is 1:4:1 so that the second diffraction grating can not be shared since if it is used in the write type, the record medium is damaged through the tracking beam.